Insulation of electrical coils in a polymerizable resin, such as various epoxy or polyester formulations, is well known. During coil wrapping or encapsulation, inevitably and unavoidably, a small number of minute voids or pockets remain in the insulation after the resin has been cured to its solid state. These minute voids are usually filled with trapped air or solvent vapor. This problem was recognized by Burke, in U.S. Pat. No. 3,240,848. Burke pointed out that at points in an encapsulated electrical device where voltage differences were great, such as between a winding and a core or between primary and secondary windings, voids at pressures less than atmospheric were particularly objectionable.
Such minute voids reduce the withstand voltage of the electrical device and can become areas of corona discharges. These corona discharges can degrade the resin insulation chemically and electrically, leading to eventual catastrophic failure of the insulation. The corona inception voltage resulting from these minute voids follows Paschen's law, wherein the lower the gas pressure inside the voids, the lower the corona inception voltage will be. If the air gap is in series with a solid dielectric material, such as an insulating enamel, the partial discharge inception voltage of the total system will be somewhat greater than the inception voltage of the gap alone.
Burke attempted to solve some of the above described problems in transformers by homogeneously incorporating various gas release agents, such as cyclohexanoic dinitrile or azo isobutyric dinitrile, with resinous insulating materials, around coils or between winding layers. These agents were heat activated after the insulating resin was cure-advanced beyond the gel stage, to release and trap nitrogen gas in voids in communication with the agent, without expanding the void volumes. However, in this process, moisture laden air originally present in the resin cross-section would appear to remain, along with residual by-products of the release agents. Thus, the nitrogen gas evolved would probably occupy only about 30 to 40 vol.% of the total voids.
Sharbaugh, in U.S. Pat. No. 4,054,680, attempted to improve the corona inception voltage of a wide variety of electrical devices, such as capacitor rolls and sheet wound transformer coils. Sharbaugh evacuated air from the electrical device in a suitable pressure vessel, and admitted selected unsaturated monomeric gases, such as vinylidene fluoride, acrylonitrile, diethylvinylsilane, cyclohexane, styrene, toluene, xylene, and benzene, into the vessel at 24 psia., to fill the evacuated voids. A voltage was then applied across conductor layers of the device, to cause the gas to chemically polymerize as a solid, to fill and eliminate any voids in the device. This method, however, could provide shrinkage problems resulting in additional voids and internal stresses. In another process, Smith et al., in U.S. Pat. No. 4,160,178, taught contacting polyacrylic-epoxy resinous insulating material with a gaas selected from nitrogen, carbon dioxide, argon, helium methane, or hydrogen, to drive off dissolved, inhibiting-effective oxygen in the resin, to initiate an anaerobic cure mechanism without the application of heat.
While Burke and Sharbaugh are effective in various degrees to help improve corona inception voltages in an electrical device, a new and improved insulation system is needed to provide not only higher corona inception voltages, but also the ability to control and extinguish any corona initiated.
There is a need for an insulation system capable of being corona free, at voltages on the order of about 500 to 1,500 volts/turn, for use in a variety of extemely sophisticated high voltage devices, such as tank enclosed, gas insulated, shunt reactors of the 1,200 kV range. These reactors may require up to 7 ft. diameter wound coils, having the characteristic of extremely high turn-to-turn voltages. In such devices and in other insulated high voltage systems, insulation with voids containing nitrogen gas is not particularly effective to provide a high level of corona extinction or corona free operation.